This invention relates to an improved thermionic energy converter and, more particularly, relates to an improved collector for a thermionic energy converter which incorporates a p-n semiconductor junction.
Thermionic energy converters offer the prospect of converting high temperature heat in the temperature range from about 1100.degree. C. to about 1900.degree. C. directly to electricity. This high temperature heat may be generated by nuclear or concentrated solar sources. In addition, thermionic energy converters offer the prospect of performing a topping conversion cycle in conventional steam electrical power plants. In any of these electrical energy generation applications it is desirable to obtain high conversion efficiencies. Generally, system efficiencies on the order of 10-20% have been obtained by utilizing a high work function emitter in conjunction with a lower work function collector. Clearly, an improvement in a portion of the system, e.g., the collector, will contribute to the overall system efficiency.
Electron transport in thermionic energy converters is typically facilitated either by using a vacuum or by creating an ionized plasma. In the former case the space charge effect is suppressed by utilizing a very close spacing between the emitter and collector. In the latter the negative space charge effect is partially or totally neutralized by positive ions in the vapor; in practice, cesium is often used because it is the most readily ionizable of all stable gases. Thus, in operating thermionic convertors, the collector will likely be exposed either to a vacuum or to an ionized plasma such as a cesium plasma.
Collectors or anodes have been fabricated from metals, from semiconductors, from oxides, from crystalline materials and from polycrystalline materials. See, e.g. I. L. Korobova, et al. "Effect of Collector Material on Characteristics of Thermal Emission Converter", Thermionic Conversion Specialists Meeting, Sept. 1-3, 1975. Eindhoven, Netherlands; D. Lieb, et al., "Thermionic Converter Performance with Oxide Collectors", 12th Intersociety Energy Conference Record, p. 1555; F. Rufeh, et al., "Collector Work Function Measurements", Thermionic Conversion Specialists Conference, 1970, p. 233. These collector materials have their individual inherent work functions which, for a given emitter material, emitter operating temperature, transport medium, and system configuration determine the system efficiency and the voltage which is generated by the converter. Under normal operating conditions a loss of energy is sustained when the electrons enter the collector due to the dissipation of kinetic energy as the impinging electrons fall from their vacuum potential to the Fermi level of the material comprising the collector, an energy dissipation per electron which is approximately equal to the work function of the collector.
It is an object of the present invention to reduce energy dissipation at the collector of a thermionic energy converter.
It is an additional object of the present invention to increase the voltage which can be obtained at the output of a thermionic energy converter.
It is another object of the present invention to use a p-n semiconductor junction in the collector of a thermionic energy generator to reduce the dissipation of electron kinetic energy as electrons impinge upon the collector.